Download t 0 (radioactive decay)

The Age of the Universe via Radioactive Decay Chronometers
Tiffany-Rose Sikorski
As the name implies, radioactive decay chronology uses the spontaneous decay of
unstable isotopes as a way to estimate the age of stars and the Milky Way Galaxy, and thus t0.
Briefly, radioactive decay is the process by which “parent” isotopes spontaneously lose
energy and turn into new “daughter” isotopes. The parent isotope’s half-life defines the rate at
which this decay occurs. Some isotopes decay too quickly to be used as chronometers. Other
isotopes decay more slowly, making them useful for estimating the age of very old objects. For
example, 232Th decays to 208Pb with a half-life of approximately 14 Gyr, on the order of the age
of the universe as estimated by other methods. Importantly, we can use changes in isotopic
abundances in meteorites and stars to estimate the age of the Milky Way and constrain t0.
Radioactive decay chronology has been especially useful in determining the age of the
solar system. In the 1950s, Claire Patterson and colleagues showed that iron meteorites contain
information about the abundances of Pb isotopes at the time the solar system formed [De Laeter,
2006]. By comparing the ratios of 207Pb/204Pb and 206Pb/204Pb in a series of meteorites and ocean
sediments, Patterson and his colleagues estimated the age of the Canyon Diablo meteorite: 4.55 ±
0.07 Gyr. Finally, Patterson and his colleagues showed that age of the Canyon Diablo meteorite
is also a good estimate of the solar system’s age. This result continues to serve as an excellent
estimate of the age of the solar system today.
Though radioactive decay chronology has been quite successful in determining the age of
the solar system, there are a number of challenges to using this approach to estimate the age of
the stars, the Milky Way, and the universe. In a meteorite, the presence of differentiated grains
provides information about initial abundances of atoms. In stars, it is more difficult to determine
the abundances of isotopes at the time the star formed, and how these abundances may have
changed over time. Chronometers formed via r-processes and with long half-lives are the most
useful for determining stellar and galactic ages.
In 1997, Cowan et al. used the ratio of Th/Eu in the sun and in the metal poor galactic
halo star CS 22892-052 to estimate the age of the Milky Way. Cowan et al. assumed that the sun
and CS 22892-052 formed with the same abundance of Th/Eu, and that this abundance changed
over time only via radioactive decay of Th. Using current measurements of the Th/Eu ratio in the
sun, as well as the known age of the sun and half-life of Th, Cowan et al. estimated the formation
abundance of Th/Eu. They then estimated the age of CS 22892-052 using the observed Th/Eu
ratio in the star and simple radioactive decay model. Their result of 15.2 ± 3.7 Gyr, sets a lower
limit to the age of the Milky Way and thus the universe. Similarly, Dauphas (2005) used the
U/Th ratio in meteorites and low-metallicity halo stars to estimate the age of the Milky Way.
Dauphas’s results, which take into account the possibility of galactic chemical evolution, suggest
that the Milky Way galaxy is 14.5 ±2.8 2.2 Gyr old.
In a summary of radioactive decay chronology studies, Spergel et al. (2003) suggest that
this approach yields a lower boundary to the age of the universe of 9.5 — 20 Gyr. These results
can serve as part of a consistency check for cosmological models. However, the radioactive
decay chronology approach is particularly limited by our understanding of the processes that
produce isotopes, our models of how isotopic abundances change over time via galactic chemical
evolution, and our ability to measure isotopic abundances of faint halo stars.
References
Cowan et al. 1997, ApJ, 480, 246
Dauphas N. 2005, Nature, 435, 1203
De Laeter, J.R. 2006, Geol. Soc., London, Spec. Public., 256, 363
Spergel et al., ApJS 148: 175-194, 2003
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